This disclosure generally relates to systems and methods for anisoplanatic imaging and beam control over an extended field of view (FOV) using phased laser arrays. In particular, this disclosure relates to systems and methods for compensating for the effects of atmospheric turbulence when projecting laser energy onto specific locations of extended objects such as orbiting objects, missiles, and airplanes.
Laser radar systems are used to transmit laser beams toward a target, receive light scattered from the target, and then process the received light to extract information about the target, such as range, velocity, vibrations, shape, materials, and surface properties such as texture or color. Laser radar systems can be used for intelligence gathering, surveillance and reconnaissance imaging, target discrimination and designation, and adaptations can be made to implement optical/laser communications, energy beaming, and directed energy weapons capabilities. However, performing these functions can be difficult when the laser beam and scattered light must pass through atmospheric turbulence, which distorts optical wavefronts and hence degrades images. Implementing these systems as phased laser arrays offers the advantages of providing reduced system size and weight while offering an architecture that scales better and degrades more gracefully with laser faults than a single large power laser source. It is these advantages that motivate the consideration of atmospheric mitigation systems for phased array lasers disclosed herein.
It is known that atmospheric turbulence-induced aberrations limit the useable aperture of telescopes. When light from a star or another astronomical object enters the Earth's atmosphere, atmospheric turbulence (introduced, for example, by different temperature layers and different wind speeds interacting) can distort and move the image in various ways. Images produced by any telescope larger than 10 cm in diameter are blurred by these distortions. The blur changes rapidly, so that in long-exposure images the higher spatial frequencies are wiped out. One way of dealing with this problem is to correct the wavefront using real-time adaptive optics. A complementary approach is to use speckle imaging techniques, in which an image is reconstructed from many short exposures.
Adaptive optics is inherently an isoplanatic technology, i.e., it assumes that the turbulence effect is constant over an angle termed the “isoplanatic angle”. In fact, the wavefront distortion due to turbulence is spatially varying due to anisoplanatic conditions, while the isoplanatic angle varies with atmospheric conditions. Adaptive optical correction of an extended region consisting of multiple isoplanatic patches (i.e., temporally coherent paths) requires significant added complexity in the adaptive optics system.
Anisoplanatism is also a concern when a high-energy laser beam is aimed at an incoming missile or aircraft for the purpose of damaging or destroying it. To accomplish this goal, it is important to deliver a maximum amount of energy density (energy per unit area) to the target, for example, by minimizing the footprint of the illuminating beam. This in turn may require that the transmitted laser beam be predistorted in a way such that it will become undistorted after it has propagated through the turbulent atmosphere to the target. Since the distortions vary with look angle, the system should be able to measure and apply predistortions over an extended range of angles.
To phase an array of lasers onto a point on a distant object moving in or above the atmosphere, an optical path difference (OPD) map must be estimated accurately for each laser to the point of interest on the object. If an array of lasers is arranged in a regular square or hexagonal array, then the array pupil is similar to a well-known Shack-Hartmann wavefront sensor configuration. It is well known that a Shack-Hartmann wavefront sensor can sense local tilts and that such local tilts can be used to reconstruct a wavefront over the entire array. If the source object is unresolved (an unresolved target is one that lies entirely within the spot size of the diffraction limited illumination beam), such a reconstructed wavefront would be appropriate for application to a phased laser array to concentrate energy onto the unresolved object.
However, when the target object is extended (i.e., the target has a size larger than the illumination beam spot size), two significant complications may arise. First, an aimpoint must be selected that is based on the resolution of the entire laser array. This poses the problem of how to create such a high-resolution image from an array of smaller subapertures, each of which has lower resolution. Second, if the object occupies multiple isoplanatic patches, then there is a different wavefront apropos for each isoplanatic patch subtended by the object. This poses the problem of how to extract the wavefront that is specific to the aimpoint. A third issue is backscatter. Some known phased array concepts rely on a single-wavelength laser, and have used various homodyne or heterodyne (i.e., coherent) detection techniques that measure complex field amplitudes. These techniques include digital holography and sheared-beam imaging. Such techniques suffer from laser backscatter that interferes with the signal returned from the target, which presents a difficulty. In addition, coherent detection techniques are more difficult than direct (incoherent) detection techniques because they typically require more coherent lasers and often require path and frequency matching of a local oscillator to a target return.
It would be advantageous to provide a phased laser array system that overcomes the aforementioned problems and is capable of high-quality anisoplanatic imaging and beam control through the atmosphere over an extended field of view.